The kind of rail tankers involved in the weekend accident that left at least 13 Canadians dead are known to puncture, according to investigators, who are busy searching for answers to explain why the train burst into flames.

The tankers are known as DOT-111 and have a history of puncturing during accidents, according to The Associated Press, citing a 1991 safety report.

The DOT-111 is common in the United States.

The steel shell is thin and can tear open in an accident, potentially spilling cargo that could catch fire, explode or contaminate the environment, the AP reported.

Investigators continue to scour the area in the town of Lac-Megantic in Quebec, with as many as 50 people still reported missing by family and friends. Some of the bodies that have been recovered are so badly burned that families are being asked to find DNA samples of their loved ones to help identify them.

Lac-Megantic, east of Montreal, was consumed by fire Saturday when a cargo train parked uphill from the town broke free, barreled toward it and derailed. The 73-car train carried up to 1 million gallons of crude oil that ignited when the train derailed, causing a fireball that engulfed the town.

The blaze destroyed much of the downtown area and landed feet away from Le Musi-Café, a packed bar that is the musical heart of the small town. Many of the missing individuals are believed to have been at the bar.

Bernard Theberje was in the bar Saturday and says he only escaped with his life because he stepped outside for a cigarette.

"We saw a big wall of fire then tried to escape. I don't think many people had time to escape," he said.

At the same time when Theberje went outside for a smoke, Luc Dion had also stepped out of the bar and says he saw a river of fire when he began running.

When asked whether he thought his friends inside the bar died he said, "Certainly. It's horrible."

Dion, a school teacher, told ABC News he believes up to 12 of his personal friends died in the blaze, including a colleague. That same school is now being used to house many of the displaced in the rural town of 6,000.

Rescue workers are still hoping to make their way through the smoldering rubble to locate the missing individuals. As of Monday, investigators could not reach the bar site because of still-smoldering hot spots.

"We have some specialized personnel from the forensics lab and hopefully we get through the whole scene in the next couple of days," Quebec provincial police Sgt. Benoit Richard said Monday.

The derailment caused fires throughout the town that devastated more than 30 buildings and sent up to 2,000 residents fleeing.

Many residents remain displaced after the fire, but authorities hope to have as many as 1,500 back in their homes by later today.

"There are still many issues that need to be resolved and we need to make this, to make sure that this process goes as safely and as smoothly as possible," Quebec Public Security officer Jean-Thomas Fortin said Monday.

Investigators have recovered two black boxes from the train since the crash and are working to determine how the crash occurred.

The railway company responsible for the train, Rail World Inc., said the train's engineer had put the proper brakes on the train when parking it uphill of Lac-Megantic. The company said that a locomotive shutdown might have released the train's airbrakes that were supposed to hold it in place overnight, setting it free on the tracks.

ABC News' Colleen Curry, ABC News Radio and The Associated Press contributed to this report.